1. Field of the Invention
The present invention relates to an adhesive containing a ladder-type polysilsesquioxane which firmly bonds metals and synthetic resins, and to an adhesive sheet having an adhesive layer formed by using the adhesive.
2. Description of Related Art
A printed circuit board with a circuit formed on the surface of an insulating substrate is widely used for electronic devices and semiconductor devices. In recent years, development of a printed circuit board with an increased circuit density and a reduced thickness is desired following the demand of miniaturizing electronic equipment and increasing functions.
A semiadditive method is known as a method of preparing a highly minute printed circuit board which realizes such a detailed circuit. This method comprises applying electroless copper plating on the surface of an insulated substrate made of a polymer and etch-removing the thin copper layer formed by electroless plating to form a circuit pattern. It is possible to form a minute circuit pattern with sufficient accuracy by this method.
However, a thin copper layer produced between an insulated substrate and a circuit pattern by electroless plating according to the semi-additive method substantially does not possess adhesiveness to an insulated substrate. Therefore, although the adhesion between the circuit pattern and the insulated substrate can be excellently maintained due to the anchor effect of the thin copper layer when an insulating substrate with a surface having a comparatively large roughness is used, adhesiveness is not sufficient enough when using an insulated substrate with a smooth surface. In such a case, the adhesion of a formed circuit pattern is insufficient.
For this reason, the surface of the insulating substrate is roughened in order to increase the adhesive strength of a circuit (Patent Document 1, etc.).
Such roughness of the surface of an insulating substrate is not likely to cause a problem when the value of the line-and-space of the formed circuit pattern is large, but may affect a high-density circuit with extra-fine lines to be used for forming a more minute circuit pattern.
As mentioned above, since an increased circuit density and a reduced thickness has been desired for printed circuit boards in recent years, a circuit-forming technique in which adhesiveness is not impaired when an insulating substrate with a smooth surface is used is strongly desired.
In relation to the present invention, a polysilsesquioxane having a ladder-type structure is known as a polymer which forms a thin film with excellent hardness, heat resistance, weather resistance, and the like (Patent Documents 2 to 5).
Patent Document 6 discloses that a resin composition containing a polyorganosilsesquioxane having a hydroxyl group, an acrylics copolymer resin having an alcoholic hydroxyl group, and a polyisocyanate compound exhibits excellent adhesion to a metal, particularly to aluminum.
However, the Patent Documents 2 to 6 do not disclose that the polysilsesquioxane having a ladder structure is used as an adhesive which firmly bonds a metal and a synthetic resin.    [Patent Document 1] Japanese Patent Application Laid-open No. 2003-49079    [Patent Document 2] Japanese Patent Application Laid-open No. 58-59222    [Patent Document 3] Japanese Patent Application Laid-open No. 7-70321    [Patent Document 4] Japanese Patent Application Laid-open No. 8-92374    [Patent Document 5] Japanese Patent Application Laid-open No. 6-306173    [Patent Document 6] Japanese Patent Application Laid-open No. 10-87834
An object of the present invention is to provide an adhesive containing a ladder-type polysilsesquioxane which firmly bonds metals and synthetic resins, and to an adhesive sheet having an adhesive layer formed by using the adhesive.
As a result of extensive studies, the inventors of the present invention have synthesized a ladder-type polysilsesquioxane by condensing polymerization of an alkoxysilane compound which has a polymerizable functional group at the molecule terminals such as 3-glycidoxypropyltrimethoxysilane. The inventors have found that a composition prepared by adding a curing agent such as 2-ethyl-4-methylimidazole to the ladder-type polysilsesquioxane can be used as an adhesive which firmly bonds metals and synthetic resins. This finding has led to the completion of the present invention.